Method of making spark plugs



Dec. 17, 1940. 1.. w. G. FLYNT METHOD OF MAKING SPARK PLUGS Original Filed June 12, 1935 INVENTOR 100/5 MG/Z Y/VT ATTORNEY Patented Dec. 17, 1940 UNITED STATES" METHOD OF MAKING SPARK PLUGS Louis W. G. Flynt, East Orange, N. 1., assignor to Breeze Corporations, Inc., Newark, N. J., a

poration of New Jersey Original application June 12, 1935, Serial No. 26,096, now Patent No. 2,083,916, dated June 15,

1937. Divided and 1937, Serial No. 147,458

Claims.

This invention relates to spark plugs and particularly to a method of making a plug which will remain gas-tight over a wide range of operating temperatures. This application is a division of 5 application Serial 26,096, filed June 12, 1935, which has now matured into Patent 2,083,916, dated J1me 15, 1937.

In the past considerable difiiculty has been experienced with spark plugs in service by reason of the fact that under high temperatures the plugs are not gas-tight. Where leaks occur the efficiency of the spark plug is materially reduced.

This is particularly noticeable where the spark plugs are used on aviation engines. The broad l5 object of this invention is to provide a method for manufacturing a spark plug which will be gas-tight and maintain its efllciency for long use under heavy stresses such as are found in aircraft practice.

One cause of the failure of spark plugs is the releasing of pressure between the sealing surfaces of the laminated mica insulation and the metal components that support them, due to relatively large differences in the thermal expansion characteristics of the two materials,

which permits the invasion of gases at high temperatures and pressure between the laminae and other contacting surfaces, the said invasion carrying with it uncombined carbon in finely divided solid form which by its electrical conductivity reduces the ohmic resistance of the insulation to a point below that established at the sparking gap provided. Therefore, a further object of this invention is to provide a spark plug the essential metallic parts of which will have thermal expansion characteristics comparable to that of the insulation material employed and thus maintain the pressure required for sealing over the entire operating temperature range.

Where mica or similar material is used as a dielectric in the construction of spark plugs there are left minute interstices between the layers of mica. In order to provide an efiicient spark plug it is desirable to fill such interstices so as to prevent a gas leak therethrough. Accordingly, a further object of the invention is to provide a method whereby a sealing agent may be incorporated in the course of the manufacture of the spark plug to fill and seal any openings or pas- 50 sages which may be present between the layers of mica or other material. Specifically, an object of the invention is to provide a method of incorporating a sealing agent in liquid form,

which sealing agent will solidify after being in- 55 corporated to eilect a permanent seal.

this application June 10,

In order to have a gas-tight spark plug over the entire temperature range encountered during service, it is necessary to provide a plug which will be gas-tight at normal temperatures and will remain so at high temperatures. Therefore, a further object of this invention is to provide a method of assembling a spark plug'in order to achieve an efiicient result. A further, specific object of the invention is to provide means to apply both an axial and a radial load to the parts in the assembly of the spark plug to efiectively seal the parts together. By providing effective sealing between the parts the service" life of the spark plug is greatly lengthened.

Overheating of the spark plug causes rapid deterioration and consequent failure. It is essential that the heat which accumulates in the spark plug be readily dissipated to prevent overheating. Therefore, a further object of the invention is to provide a spark plug so constructed as to permit the rapid dissipation of accumulated heat.

- A specific object of the invention is to provide a construction wherein a single metallic member is in direct contact with the outer portion of the spark plug to permit direct conduction of heat from the center electrode of the spark plug to the outer casing. A further specific object of the invention is. to provide a spark plug in which the cross-sectional area of the center electrode is reduced in the upper portion so as to retard the transmission of heat to the upper part of the spark plug, thereby minimizing the temperature at the point where the contact between the conductor cable and the spark plug terminal is made in order that temperatures damaging to the cable insulation may not be transmitted to that point.

Where radio equipment is used in proximity to an internal combustion engine it is necessary to shield the ignition system by surrounding the system with grounded conductive casings to prevent interference with the radio equipment. This shielding must extend to and include the spark plugs. 'Therefore, a further object of the invention is to provide a spark plug adapted to receive radio shielding elements.

The invention consists of the construction,

combination and arrangement of parts and the steps of the method, as herein illustrated, described and claimed.

In the accompanying hereof, is illustrated one the invention, in which drawing, forming part form of embodiment of drawing similar reference characters designate corresponding parts,

and in which:

Figure 1 is an enlarged vertical section through the spark plug;

Figure 2 is an enlarged vertical section through the sealing bushing and compression hoop before the application thereof to the sparkplug;

Figure 3 is an enlarged horizontal section taken approximately on line 3-3 of Figure 1, looking downward; and,

Figure 4 is a bottom plan view, on a smaller scale, of the plug shown in Figure 1.

Referrin to the drawing, the outer part of the spark plug comprises a shell 6 provided with external screw-threads I for engagement in an appropriate opening in a cylinder head. The lower part of the shell 6 is formed with an inwardly curved base 8 having formed therein a plurality of slots 9 to provide a plurality of sparking points or areas II]. The shell 6 is provided with an internal shelf or shoulder II and the upper part of the shell 6 is provided with internal threads I2 to receive a retaining means as hereinafter described. A washer I3 may be disposed adjacent the threads I to insure a. gastight seal between the shell 6 and the cylinder head of the engine which receives the spark plug.

Disposed in the shell 6 is a center electrode I4 provided with a tip I5. The electrode I4 is formed of a metal having an exceedingly low or practically negligible co-efiicient of expansion and the tip I5 maybe formed of metal adapted to withstand corrosionand erosion in service. The electrode I4 and the tip I5 may be made of a single piece of metal having a negligible co-efficient of expansion rather than of two pieces. The electrode I4 may be coated with a material of greater heat emissivity than the metal of the electrode I4, to more readily permit conduction of heat from the electrode I4 to the outer part of the spark plug. The tip I5 is formed with a shoulder I6 to receive the sealing means herein after described.

Disposed around the electrode I4 and the upper portion of the tip I 5 is a mica insulating tube I! which may be formed from a single sheet of mica wrapped several times around the electrode I4. Around the lower part of the tube I1 is disposed a stack of mica washers I8 which rest on the shoulder I8 of the tip I5. Centrally of the plug and above the washers I8 is disposed a sealing bushing I9 formed with an integral fiange 20 having a tapered face 2| adapted to bear against the internal shoulder I I of the shell 6. The bushing I9 may be made of copper or other metal suitable for rapid conduction of heat.

Surrounding the bushing I9 is a compression hoop 22 formed of a metal having a negligible coefiicient of expansion similar to the electrode I4. The lower portion of the hoop 22 is formed with a flange 23 overlying the flange 20 of the bushing I9. 7

Above the bushing I9 and hoop 22 and surrounding the tube I! is a second stack of mica washers 24. Above the washers 24 and surrounding th upper end of the electrode I4 is a terminal 25 provided with an annular groove 26 to receive a contact means to supply current to the electrode I4. The upper portion of the electrode I4 is formed with a bore 21 which extends to a point adjacent the top of the bushing I 9 The purpose of the bore 21 is to reduce the crosssectional area of the upper portion of the electrode I4 to retard the passage of heat to the upper part of the spark plug. When the spark plug is assembled the top or the electrode I4 is turned over at 28 against the terminal 25 to hold the parts together as hereinafter described. A compressible washer 29 is disposed on the flange 23 of the hoop 22 and the electrode II with its associated parts is held in the shell 6 by a retaining cap 30 having a lower threaded portion 3| adapted to engage with the internal threads I2 of the shell 6. The lower part of the retaining cap 30 bears against the washer 29 which serves the purpose of equalizing the pressure at all points on the flange 23. The upper portion of the retaining cap 30 is formed with threads 32 to provide for the optional application to the spark plug of a radio shielding cap.

In the manufacture of the spark plug the electrode I4 and th tip I 5 are first formed. If a two piece construction is used, the tip I5 is welded to the electrode I4 and the parts are machined to exact size to form the shoulder I 6. If a one piece construction is desired, the electrode I4 and the tip I5 are machined directly from a single blank. The bore 21 is then formed in the electrode I4 which is now ready to receive its associated elements.

A sheet of mica is then wrapped around the electrode M to form the tube l1 and the lower stack of mica washers I8 is applied thereover. The sealing bushing I9 and the compression hoop 22 are next applied and when first applied to the electrode I4 th bushing I9 and the hoop 22 are in the shape shown in Figure 2. The two members may be applied as a sub-assembly or may be applied separately. In either case, before application, the bushing I9 and the hoop 22 are stressed outwardly by means of an expanding drift so that when these members are reduced by swaging, as hereinafter described, there will be no tendency on the part of the metal to again expand.

Where a single sheet of mica is used to form the tube I! there are left two openings I'Ia longitudinally of the tube I'I. These openings are substantially triangular'in cross-section and. dependent in size on the thickness of the mica. In Figure 3 the openings IIa are exaggerated to more properly explain their presence. Where more than one sheet of mica is used to form the tub I I the number of openings I'Ia will be multiplied. The openings will appear adjacent the edges of the sheets of mica and also where a succeeding sheet overlaps a preceding sheet. These openings I-Ia will permit the escape of gas and it is desirable that they should be filled and sealed. After the application to the electrode I4 of the tube I1, the stack of washers I8 and the bushing I9, but before the application of the upper stack of washers 24 and the cap 25, the openings I la are filled with a suitable sealing agent.

Various compositions may be used for the sealing agent, but extremely satisfactory results have been achieved by the use of Glyptal. Glyptal" is a synthetic resin having an extremely high dielectric strength. It will liquefy at approximately 350 F. and is very viscous in its liquid form.

. constrict the tube I1 around the electrode I4.

The purpose of the initial swaging is to prevent an excess amount of sealing agent being forced in the tube ll.

into the openings Ila below the flange 20 of the bushing Hi. In order to have effective sealing it is not necessary that the openings I'Ia be filled with the sealing agent below the flange 20 as a surplus of the sealing agent below the flange 20 would induce improper carbonization of the sealing agent under the extreme heat to which the lower part of the plug is subjected.

The electrode l4 and its associated parts may be inserted in an open-ended cylinder and the sealing agent isinjected under considerable pressure. The sealing agent is first liquefied by heating and the cylinder or other fixture may be heated to maintain the sealing agent in liquid form until the same is thoroughly incorporated The sealing agent will not only fill the openings Ila, but any free spaces between the laminations of the tube l1, and the air in such spaces will be exhausted through the lower end of the cylinder.

Thereafter, the upper stack of mica washers 24 is applied around the upper portion of the tube l1. The washers 24 are preferably saturated in a solution of Glyptal before being applied to the tube H. The cap 25 is then applied and the assembled parts are placed in a suitable fixture. I An axial load is imposed on the entire structure between the cap 25 and the tip l5 so that the mica washers 24 and I8 are securely pressed together. While the axial load is maintained the upperportion 28 of the electrode I4 is turned over as shownin Figure 1 to securely lock the parts in permanent relationship.

The bushing l9 and the hoop 22 are then reduced by passing the entire assembly through a swaging die. The flange 20 on the bushing i9 is slightly tapered as shown at 2| in Figure 2, so that the assembly may be readily passed through the swaging die without any drag by reason of the outer edge of the-flange 20. The application of the axial pressure and the radial pressure by reason of the swaging will extrude any surplus quantities of the sealing agent which has been incorporated in the mica laminations. In addition, the radial load causes a slight elongation of the bushing l9 and hoop 22, thus increasing the axial pressure on the parts and effecting a permanent gas-tight seal.

Since the electrode l4 and the hoop 22 are made of materials having a negligible co-eflicient of expansion, there is no possibility of leakage during service by reason of the expansion of the parts. Furthermore, since the bushing l9 and hoop 22 have been initially stressed outwardly before swaging, these parts are in a normal rather than stressed condition, so that there is no tendency to expand under the heat of. service.

Following the foregoing operations the washers 24 and I8 are machined to the contour shown in Figure 1 to present -a smooth surface, and the bushing l9 and hoop 22 are also machined ,from the shape shown in Figure 2 to the shape shown in Figure 1. The electrode l4 and its associated parts are now mounted in the shell 6 and held in place by the retaining cap 30 in the usual manner. The spark plug is now ready to be put in service. The following claims are directed only to the method of constructing the plug, the structure of the plug per se being claimed in the aforementioned application Serial No. 26,096.

Having thus fully described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. The method of making a spark plug which consists'in forming a center electrode, placing insulation around the electrode, expanding a sealing bushing and compression member and applying said scaling bushing and compression member around part of the electrode, applying axial pressure ,to seal the parts together, applying radial pressure to further seal the parts,

machining the outer surface of the assembled parts, and inserting the parts in a holding shell.

2. The method of making a spark plug which consists in forming a center electrode, placing insulation around the electrode, applying a sealing bushing and a compression member around part of the electrode, applying radial pressure to partially seal the parts, incorporating a sealing agent to fill any spaces in the insulating material, applying axial pressure to seal the parts together, applying additional radial pressure to further seal the parts and to increase the axial pressure, machining the outer surface of the assembled parts, and inserting the parts in a holding shell.

3. The method of making a spark plug which consists in forming a center electrode, wrapping mica insulation about the electrode, applying a sealing bushing around the mica, initially swaging the bushing to control the flow of a sealing agent, injecting a liquid sealing agent longitudinally of the mica wrapping, applying axial pressure to seal the parts together, applying further radial pressure, machining the outer surface of the assembled parts, and inserting the parts in a holding shell.

.4. The method of making a spark plug which consists in forming a center electrode, placing insulation around the electrode, applying a seal- 1 ing bushing around part of the electrode, applying axial pressure to seal the parts, and subsequently applying radial pressure to further seal the parts and to elongate the bushing to increase the axial pressure.

5. The method of making a spark plug which consists in forming a center electrode, placing insulation around the electrode, applying a, sealing bushing and a compression member around part of the electrode, applying axial pressure to seal the parts, and subsequently applying radial pressure to further seal the parts and to elongate the bushing and compression member to increase the axial pressure.

- LOUIS W. G. FLYNT. 

